From 275bf960ac69744430a6725a4ed7f50d36cf1441 Mon Sep 17 00:00:00 2001 From: Jason Wang Date: Wed, 18 Jan 2017 15:02:01 +0800 Subject: [PATCH] vhost: better detection of available buffers This patch tries to do several tweaks on vhost_vq_avail_empty() for a better performance: - check cached avail index first which could avoid userspace memory access. - using unlikely() for the failure of userspace access - check vq->last_avail_idx instead of cached avail index as the last step. This patch is need for batching supports which needs to peek whether or not there's still available buffers in the ring. Reviewed-by: Stefan Hajnoczi Signed-off-by: Jason Wang Acked-by: Michael S. Tsirkin Signed-off-by: David S. Miller --- drivers/vhost/vhost.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index d6432603880c..9f118388a5b7 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq) __virtio16 avail_idx; int r; - r = vhost_get_user(vq, avail_idx, &vq->avail->idx); - if (r) + if (vq->avail_idx != vq->last_avail_idx) return false; - return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx; + r = vhost_get_user(vq, avail_idx, &vq->avail->idx); + if (unlikely(r)) + return false; + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); + + return vq->avail_idx == vq->last_avail_idx; } EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);